Device for producing bursts of charged particles

ABSTRACT

Device for producing bursts of charged particles issuing from an accelerator of the type in which the beam is alternatively directed onto a target, said active target, and onto a nonreactive target in a shifted position with respect to the active target, under control of a beam deflecting device that comprises essentially two members symmetrically disposed with respect to an axis: a plate parallel to said axis and brought to a reference potential and a plurality of identical small plates parallel to said axis, regularly distributed along it and brought to a bias potential, and sections of coaxial cable connecting the adjoining small plates whose the length is so chosen that the pulses propagating through the second member has the same propagating velocity as the beam.

United States Patent [72] Inventor Michel Roche Quetlgny, France [21]Appl. No. 769,857

[22] Filed Oct. 23, 1968 [45] Patented Dec. 21, 1971 [73] AssigneeCommissarlat A LEnergie Atomique Paris, France [32] Priority Oct. 31,1967 [3 3 France [54] DEVICE FOR PRODUCING BURSTS OF CHARGED [56]References Cited UNITED STATES PATENTS 3,174,070 3/1965 Moulton 315/18 X3,280,361 10/1966 Goldberg et al. 315/3 3,376,464 4/1968 Loty et al315/3 3,400,298 9/1968 Hans-Joachim Krahn 315/35 PrimaryExaminer-Richard A. Farley Assistant Examiner-Brian L. RibandoAttorney-Craig, Antonelli, Stewart & Hill ABSTRACT: Device for producingbursts of charged particles issuing from an accelerator of the type inwhich the beam is alternatively directed onto a target, said activetarget, and onto a nonreactive target in a shifted position with respectto the active target, under control of a beam deflecting device thatcomprises essentially two members symmetrically disposed with respect toan axis: a plate parallel to said axis and brought to a referencepotential and a plurality of identical small plates parallel to saidaxis, regularly distributed along it and brought to a bias potential,and sections of coaxial cable connecting the adjoining small plateswhose the length is so chosen that the pulses propagating through thesecond member has the same propagating velocity as the beam.

PATENTEU B5021 1911 3629.643

FIG. I

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FIG.4

ATTORNEYS DEVICE FOR PRODUCING BURSTS OF CHARGED PARTICLES The presentinvention relates to a device intended for the time modulation of a beamof charged particles.

Such devices have been constructed wherein a beam of charged particlesis alternatively directed onto an axial active target and onto anonreactive target in a nonaxial position. Such a result is obtained byapplying to the beam a pulsemodulated electric field whose direction isperpendicular to the usual direction of the beam. To this end, it hasbeen proposed to direct the beam between two deflecting plates fed witha square wave voltage. For improving the rising edge of the so-formedpulses, it has been also proposed to make use of a delay line throughwhich the pulse edge propagates with the same velocity as the particles:for instance, a helical travelling waveline is disposed within ametallic cylinder through which the beam is longitudinally directed;said helical line has its axis parallel to the usual direction of thebeam, but the latter does not pass through it, and a square wave voltageis applied between the line and the metallic cylinder acting as anelectrical ground terminal.

Such a process makes it possible to make an apparatus whose travellingwaveline comprises a helix surrounded with a metallic box and has a highimpedance so that the pulse generator connected to it delivers a weakcurrent. Unfortunately, the above-described circuit does not make itpossible to obtain beam pulses with sufficiently short rise-times.

The object of the present invention is to provide a device of theabove-described type that makes it possible to obtain beam pulses whoserise-times are very short.

The device according to the invention intended for producing bursts ofcharged particles is characterized in that the device for deflecting thebeam onto a nonreactive target or a diaphragm comprises essentially twomembers symmetrically disposed with respect to an axis: one plateparallel to said axis and brought to a reference potential, and aplurality of identical small plates parallel to said axis, regularlydistributed along it and brought to a bias potential, the small platesbeing connected through sections of coaxial cable having each apredetermined length for delaying the pulses transmitted through theassembly of small plates and cable sections, of a time interval equal tothe one taken by the beam for passing between corresponding points onadjacent small plates. Preferably, the extremity of the first smallplate opposed to the target is connected to a generator of pulses whosethe amplitude is equal to the difference between the bias potential andthe reference potential.

Inasmuch as the large transmission range of a coaxial cable may be equalto 1,000 ml-lz. and the insertion of the small plates in the coaxialstructure results in only a light loss of performance, the rise-times ofthe produced bursts of particles can have a duration in the range of lnsec.

The invention will be now described by way of a nonlimitative examplewith reference to the accompanying drawings in which:

FIG. 1 shows a device according to the invention for deflecting a beamof charged particles in an accelerator,

FIGS. 2 and 3 are graphs for illustrating the variations in time of thevoltage U applied between the plate and the deflecting small platesandthe ones of the current; I of the active target, and

FIG. 4 is a schematic diagram of a generator for use with the device ofFIG. 1.

The FIG. 1 illustrates the application of the invention to a low-energyelectrostatic accelerator. The deflecting device 6 is intended to directthe beam alternatively onto a nonreactive target 8 or diaphragm (path11) and onto an active target 4 (path 2).

The deflecting device comprises two members symmetrically disposed withrespect to the axis of the beam: a long plate brought to thereferencepotential and a plurality of small plates 12,, 12,, 12,, 12 regularlydistributed along the axisof propagation of the beam and brought to thebias potential +A.

The small plates are interconnected through the inner conductor ofsections of coaxial cable 14,, I4 14 14 The length of each section ofcoaxial cable is chosen for delaying the pulses transmitted by theassembly of small plates 12 and of cable sections 14, of a time intervalequal to the one taken by the beam for crossing the space betweencorresponding points on two small plates.

The first section of coaxial cable 14 is connected to a pulse generator16 (not shown) while the last section 14 is connected to a matched load18 (not shown).

In operation, the voltage applied between the plate 10, on one hand, andthe assembly of small plates 12, on the other hand, is equal to the sumof two components: one of them is the bias voltage +A applied to thesmall plates 12, the other is the voltage corresponding to the amplitudeof the pulses issuing from the generator 16. The absolute amplitude ofsaid pulse is equal to the one of the bias voltage, but the polarity isreversed.

As a result, a positive voltage is applied to the plates during the timeinterval separating two pulses. The beam is therefore deflected andstopped by the nonreactive target or diaphragm 8. On the contrary,during every pulse, the voltage applied to the plates falls to zero: thebeam is no longer deflected and reaches the active target 4. It will benoted that the bias voltage can be adjusted for allowing the beam toreach the center of the active target during the pulse.

The FIG. 2 illustrates the variations with time of the voltage U appliedbetween the plate 10 and the small plates 12, and the FIG. 3 illustratesthe variations of the ionic current on the active target in the sametime.

The transmission range of the assembly of small plates 12 and of coaxialcables 14 is, of course, slightly reduced, but the rise-time of thetransmitted pulses is kept at a value approximating l nsec. and remainsattractive for the application under consideration.

In other respects, the impedance presented to the generator 16 is verylow, which conduces to the design of a high-power pulse generator.

The applicant has constructed a deflecting system of the described type.The small plates l2 are placed at a distance equal to about one-third oftheir length. They are held in the correct position by means of supportsmade of a plastic material having a low-dielectric constant. Thesections of coaxial cables are long with respect to the sizes of a smallplate; the characteristic impedance of the cable is comprised betweenand D.

The generator comprises essentially an electron multiplying tube 15whose the output current controls the grid of an anode follower tetrode18 with a relatively low-anode load constituted by an insulatingcable-transformer 17, as seen in FIG. 4.

Such a device is the most attractive with a neutron producing target forit is possible to obtain a pulsed beam by interaction and, from saidtarget, bursts of neutrons with very short edges.

For instance, riseand fall-times of 1.4Xl0 s are easily obtained andeasily reproducible.

It is understood that the present invention is not limited to thedetails given herein and that it may be modified within the scope of theappended claims.

What we claim is:

1. A device for producing bursts of charged particles from a particlebeam issuing from an accelerator, in which successive sections of thebeam are alternately directed onto an active target and onto anonreactive target positioned at a shifted position with respect to theactive target, under control of a beam deflecting device, said beamdeflecting device comprising two members symmetrically disposed withrespect to an axis, one member being formed by a plate parallel to saidaxis and brought to a. reference potential, and the other member beingformed by a plurality of identical small aligned plates disposedparallel to-said axis in regularly spaced relation and brought to a biaspotential, said small plates being connected through sections of coaxialcables having each a predetermined length for delaying pulsestransmitted through the member formed of the small plates and the cablesections, said time delay of said cable sections being equal to the timetaken by the beam for passing between corresponding points on ad- 5jacent small plates.

2. A device according to claim 1, for producing bursts of chargedparticles issuing from an accelerator, in which the extremity of thefirst plate, opposed to the target, is connected to a pulse generator.

3. A device according to claim 2, for producing bursts of chargedparticles issuing from an accelerator, in which the generator generatespulses with an amplitude equal to the difference between said biaspotential and said reference potential.

4. A device according to claim 2, for producing bursts of chargedparticles issuing from an accelerator, in which the pulse generatorcomprises an electron multiplying tube and a tetrode whose load isconstituted by an insulating cable-transformer.

1. A device for producing bursts of charged particles from a particlebeam issuing from an accelerator, in which successive sections of thebeam are alternately directed onto an active target and onto anonreactive target positioned at a shifted position with respect to theactive target, under control of a beam deflecting device, said beamdeflecting device comprising two members symmetrically disposed withrespect to an axis, one member being formed by a plate parallel to saidaxis and brought to a reference potential, and the other member beingformed by a plurality of identical small aligned plates disposedparallel to said axis in regularly spaced relation and brought to a biaspotential, said small plates being connected through sections of coaxialcables having each a predetermined length for delaying pulsestransmitted through the member formed of the small plates and the cablesections, said time delay of said cable sections being equal to the timetaken by the beam for passing between corresponding points on adjacentsmall plates.
 2. A device according to claim 1, for producing bursts ofcharged particles issuing from an accelerator, in which the extremity ofthe first plate, opposed to the target, is connected to a pulsegenerator.
 3. A device according to claim 2, for producing bursts ofcharged particles issuing from an accelerator, in which the generatorgenerates pulses with an amplitude equal to the difference between saidbias potential and said reference potential.
 4. A device according toclaim 2, for producing bursts of charged particles issuing from anaccelerator, in which the pulse generator comprises an electronmultiplying tube and a tetrode whose load is constituted by aninsulating cable-transformer.